IPT systems, and the use of a pad which includes one or more windings which may comprise the primary or secondary windings for inductive power transfer, are disclosed in our published international patent application WO 2008/140333, the contents of which are incorporated herein by reference.
One particular application of IPT power transfer pads is electric vehicle charging, and that application is discussed in this section to provide the background to one application of the invention. However, electric vehicle charging is an example of only one application, and the invention has application to inductive power transfer in general. Electric vehicle charging may occur while the vehicle is stationary, or alternatively while the vehicle is moving along a roadway, for example. IPT power transfer pads can be used both in the vehicle as a power “pickup” (i.e. the secondary side winding of the IPT system), and at a stationary location such as a garage floor or a roadway for example as the “charging pad” (i.e. the primary side winding) from which power is sourced.
The purpose of an IPT roadway system is to wirelessly transfer power to a stationary or moving vehicle without physical contact to the vehicle. The transmitting part of the system consists of a power supply supplying a lumped coil (for example a pad as described above) or a track with many similar lumped coils where such a system is tuned for operation at a suitable frequency, usually anywhere from 10 kHz to 150 kHz. Where the receiver is placed underneath a vehicle and coupled to receive power either when the vehicle is stationary above or near (in sufficiently close proximity to couple power) to the primary transmitter. The pickup receiver also typically comprises a lumped coil (such as a pad described above) which is connected to a converter and appropriate controller within the vehicle to regulate power. For convenience, the part of a roadway from which power may be received inductively is referred to herein as a track.
The track may be formed by placing a plurality of pads along the centre of a lane in a roadway. This results in the possibility of an essentially continuous supply of power to the vehicle as it moves along the roadway in the immediate vicinity of the track.
In recent years such systems have received increasing attention due to their potential to allow sustainable wireless powered personal transportation. For such a system to be useful it must not only be able to transfer sufficient power over an airgap of reasonable size (e.g. 100-300 mm) it must also prove tolerant to any displacements between track and pickup, to avoid dependency on a vehicle-to-track guidance system. In a roadway system such displacement will most likely occur in the lateral direction (orthogonal to both vertical and the direction of movement) for moving vehicles. For stationary vehicle charging the ability to transfer acceptable levels of power with suitable longitudinal displacement is of particular concern in order to ensure ease of parking. The power transfer profile in the pick-up pad is ideally a smooth power profile which is essentially constant (and sufficient) over as wide as possible a distance laterally, with smooth drop-offs at each end. Such a power transfer profile eases the demands on the electronic (primary and secondary) regulators in the system, enabling improved operating performance for a comparable coupling over a system where during operation significant variations are experienced in the coupling between the primary and receiver pads.
Referring to FIG. 1, a magnetic flux pad construction previously disclosed by Boys, Covic, Huang and Budhia is shown which has excellent characteristics suitable for vehicle applications. The construction of FIG. 1 has been published in international patent publication WO2010/090539A1. For convenience, this general construction is referred to herein as a DDP pad, and is generally referenced DDP in the relevant drawing figures in this specification.
The DDP pad shown in FIG. 1 generally comprises two substantially coplanar coils referenced 2 and 3 which are magnetically associated with and sit on top of, a core 4. As can be seen from the drawing figure, the core 4 may consist of a plurality of individual lengths of permeable material such as ferrite strips or bars 5 which are arranged parallel to each other but spaced apart. The pad construction may include a spacer 6 on which the core is located, and a plate 7 below the spacer. In some embodiments a cover 8 may be provided on the other surface of the flat coils 2 and 3. Padding 9 may be provided about the periphery of the pad. As can be seen, the coils 2 and 3 each define a pole area 10 and 11 respectively. This DDP pad construction as shown in FIG. 1 is a polarised pad that shows very good characteristics suitable for IPT power transfer applications such as vehicle charging. The coils 2, 3 may be connected in series but electrically out of phase and driven by a single inverter to produce a stationary time varying magnetic field to couple to a receiver (which may for example be of substantially the same magnetic design) at distances suitable for electric vehicle power transfer with good coupling.
Turning now to FIG. 2, the DDP construction of FIG. 1 is shown but further including a quadrature coil 12 (referred to herein as a DDPQ pad). This construction is also described in patent publication WO2010/090539A1. The quadrature coil extends the power transfer profile when there is lateral movement of the construction shown in FIG. 2 with respect to a flux generator such as the DDP pad of FIG. 1 when energised by an appropriate inverter. The quadrature coil allows power to be extracted from the “vertical” component of the magnetic field that the receiver pad intercepts while the other coils 2, 3 facilitate power extraction from the “horizontal” component of the flux intercepted. Therefore, the construction of FIG. 2 is suited as a flux receiver.